Ice storage Insight for Energy Modelers

There are a lot of good reasons. This article focuses on a few of the main red flags telling you to get your butt in gear and get some ice tanks. And, if LEED is important to you, we will talk about how ice storage can yield big LEED points.

But before we talk about ice storage, let’s start with an ice-breaker: What is the number one reason to use ice storage? Now, if you said 'to save energy', you are WRONG. Would it surprise you to know that reducing energy consumption is not the most common reason to use ice storage?. The real reason to use ice storage is that IT SAVES EVEN MORE MONEY THAN IT SAVES IN ELECTRICITY. Example: you can "charge" the tanks at night, and nighttime electrical purchases, which are increasing in renewable energy content, can save you tons of money, and easily earn you additional LEED points. Ice storage often USES LESS SOURCE ENERGY, and thus has a REDUCED CARBON FOOTPRINT..

So, why is it so good for the environment? To answer the question, remember, not all energy is created equal. Ice storage helps optimize the use of utility companies, so even though it costs some energy on the building meter, the utility company itself can operate more efficiently, and operate with less overhead since ice storage uses energy at a time when it is more available. Therefore, they deliver you the energy at a lower cost, and they have less infrastructure (big picture energy savings).

Now, back to the other question: what is a red flag telling you to get your butt in gear and to use ice storage? Let's cover 2 common reasons:

With ice storage, a piece of cooling equipment can be sized smaller, keeping ice as backup for the few hours here and there when it needs additional capacity. The purpose in this case is to lower the up front cost of equipment. (For instance, a building in Bangor Maine reaches 100 tons for a few hours in July. However, its load is rarely over 70 tons. So, rather than buy a 100 ton unit, buy a 60 ton unit, and some ice storage bins)

Electric demand is typically 50% of an electric bill. With as much as 50% of the demand coming from cooling equipment, ice storage is used to shift the cooling equipment demand to a much more affordable time of day. If a building has an electric demand ratchet, a rough estimate tells us that this could save 25% of the building cost.

More often than not, the ice storage is used both as a demand savings and as a method to use smaller equipment. As a result, the annual cost can be lowered without a significant increase in star-tup cost. Of course, an increase in start-up cost can pay back dividends.

So, what is the number one red flag that ice storage is a good idea? The answer: a big difference in on-peak and off-peak demand. Say, if there is a $10 difference between the two, and there are at least eight off peak hours per day (to charge the tanks), it is definitely worth doing a simulation, as their is a high probability that it will pay back, even in climates with a short summer season. Now, if your utility rate has a ratchet, it will probably pay back even faster.

So, how do we simulate an ice storage project? The nice part about an ice storage simulation is that it is comparative, meaning, the virtual building does not have to be super detailed. So, it isn’t necessary to spend hours modeling.

What do you need to model ice storage?

General Building Parameters

Exact rate structure

Time of day schedule for one year, including peak season if it exists

Plant details in both cooling mode and ice making mode

Ballpark idea of what you wish to accomplish

Of the several things needed, the time of day schedule affects your cooling plant the most. It determines when to make ice, when to melt ice, and when to hold ice. In a simple time of day schedule- where it is on peak during the day, and off peak at night - start by simply charging during off peak, and discharging during on peak hours. Some software, such as Trace 700 has a feature that will minimize on peak demand when melting ice, through optimized ice storage. The on peak/off peak schedule is simple for the most part, but what about when weekends are all off peak? It depends if the weekend is occupied or not. However, if there are standby losses, it is foolish to charge all weekend. Instead, discharge any remaining ice to meet the cooling load on Saturday, and let the tanks sit until Sunday afternoon. It may be a good idea to start charging the tanks early enough on Sunday, so they are at maximum capacity Monday morning, when they are needed most.

While coinciding schedules are key to ice storage success, there are several other factors. For instance, what if there are only seven off peak hours? Considering that ice making usually lowers a plants total capacity, it may prove impossible to fully charge the tanks in a short period of time. So, the tank sizing is largely dependent on the off peak capacity multiplied by the number of hours minus the number of cooling ton-hrs needed at night (unless a second unit does cooling). If ample capacity is available, tank sizing is determined largely based on startup cost. Otherwise, the target capacity can be obtained by looking at the on peak time period, and the average hourly tonnage. From that information, it is possible to get the ton-hrs. A final variable to keep in mind: some hours have low loads. During those hours, it may be valuable to hold off on ice melting, as they are likely not peak hours. It is best to experiment with an energy-model a bit to make the most informed decision.

If you would like to see an applied example, a Trace 700 ice storage example file is available for free. Included with it, is a .pdf that will help guide you through an ice storage setup in Trace 700.

About the Author

Robert Fassbender

Bob Fassbender graduated from the University of Wisconsin - Madison with a degree in Chemical-Engineering. Following graduation, he spent 3 years working as a Marketing Engineer for Trane C.D.S. In the C.D.S. group, Bob developed and supported design and analysis software, primarily TRACE 700™. In addition to his development work, Bob also traveled around the country as a TRACE 700™ and System Analyzer™ instructor. Bob is also an experienced user with eQUEST energy modeling software. Today, Bob continues training and energy modeling as a LEED accredited professional (with a focus on LEED EA credit 1).

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